This invention relates to a semiconductor optical device which are applicable for an optical communication, an optical disk device and an optical interconnection and the like and a manufacturing method thereof, and in particular, to the semiconductor optical device which has a spot-size conversion function and a manufacturing method thereof.
In a semiconductor optical device, such as, a semiconductor laser, a semiconductor optical amplifier and a semiconductor optical modulator, a spot diameter of an optical beam which is emitted from an optical waveguide is small and further, a beam divergence is large. Consequently, it is generally difficult to couple the semiconductor optical device to an optical fiber or a silica-based optical waveguide.
To this end, the semiconductor optical device is conventionally coupled to the optical fiber or the optical waveguide by the use of an optical module with a lens. However, the lens if generally expensive, and further, the position of semiconductor optical device must be adjusted with parts, such as the lens, the optical fiber and the optical waveguide at a high accuracy. This remarkably increases the price of the optical module.
In this event, if the optical spot-size is enlarged at a facet of the semiconductor optical device and further, the beam divergence becomes narrow, the semiconductor optical semiconductor device can be coupled to the optical fiber at the high efficiency without the positional adjustment due to the high accuracy by the use of the expensive lens. Consequently, it may be possible to largely reduce the price of the optical module.
From the above-mentioned reasons, various suggestions has been conventionally made about the semiconductor optical device having the spot-size conversion function.
For instance, suggestion has been made about a semiconductor optical device of the Fabry-perot laser (thereinafter, referred to as FP-LD, and called a first conventional reference) in Japanese Unexamined Publication No. Hei. 7-283490. In this FP-LD, the waveguide of the semiconductor is integrated to convert the optical spot-size. The FP-LD has a gain region and a spot-size conversion region on a semiconductor substrate. With such a structure, the spot-size is enlarged by changing the layer thickness of the optical waveguide to realize a narrow beam divergence in the above spot-size conversion region.
On the other hand, another suggestion has been made about another FP-LD (thereinafter, called a second conventional reference) in Electronics Letters August 1996, Vol. 31 No. 17, pp. 1439-1440. In FP-LD of the second conventional reference, the optical spot-size is enlarged at the laser output facet by the use of a lateral direction taper shape. In this event, the lateral direction taper shape is formed by etching an epitaxial layer which is flatly grown on the entire surface of the substrate without using the selective growth method.
In the first conventional reference, the spot-size conversion region (namely, an active region) must be formed within the length between 200 .mu.m and 300 .mu.m. Consequently, the device yield for each wafer is reduced. Further, the photo-lithography steps must be twice carried out to form the selective growth mask, and the mesa-etching process must be also performed to form the waveguide. As a result, the manufacturing process inevitably becomes complicated.
On the other hand, the above-mentioned problem may be solved because the lateral taper shape having the optical gain is formed by etching the semiconductor active layer in the second conventional reference. However, the semiconductor layer must be processed in a sub-micron order at the tip portion of the tapered waveguide. Consequently, it is difficult to form the waveguide at the high accuracy by the use of the dry method in addition to the wet method. As a result, it is also difficult to uniformly fabricate the taper shape and to excellently reproduce the device characteristic.
Moreover, the device characteristic including the beam divergence largely depends upon the stripe width of the active layer in the first conventional reference. Consequently, it is difficult to stably fabricate the device having the narrow beam divergence on the condition that the excellent characteristic, the reproducibility and the uniformity of the shape are kept.
Further, the process accuracy is slightly increased in the dry method as compared to the patterning due to the wet method to form the optical waveguide. However, the active layer is damaged from the side surface in this case. Moreover, it is difficult to excellently form a buried layer at the side surface of the optical waveguide layer during growing the buried layer which is carried out after patterning the optical waveguide layer. Consequently, it is also difficult fabricate the device at a high reliability.
To avoid this problem, the wet process must be carried out to remove the damaged layer after the dry-etching process. Finally, the high process accuracy can be practically obtained.
Further, when the coupling with the optical fiber is taken into account, it is desirable that the optical spot is formed into an approximately circular shape at the output facet with the small emission angle. However, the circular spot shape is realized only by changing the layer thickness like the first conventional reference or by forming the lateral taper shape like the second conventional reference. Consequently, the design flexibility of the device parameters, such as, the active layer structure, the active layer width and the taper shape, is remarkably restricted.